The present invention relates to an amplifier circuit having a signal detection function, which is preferably used in an optical receiver.
In an optical receiver used in an optical communication system, an optical signal from an optical fiber is converted to an electric current by a photodiode and then converted to a voltage signal by a transimpedance amplifier. However, this voltage signal is very small in the vicinity of a minimum light reception level of an optical input and therefore needs to be amplified by an amplifier circuit.
The first conventional technique employs a feed-forward amplifier circuit including a first-stage amplifier for amplifying a very small signal and a capacitor-retained reference voltage generation circuit for generating a reference voltage according to an output signal of the first-stage amplifier and retaining the reference voltage in a capacitor. This feed-forward amplifier circuit has a structure wherein an output of the reference voltage generation circuit is input to an amplifier of the next stage as a reference voltage (see M. Nakamura et al., “An Instantaneous Response CMOS Optical Receiver IC with Wide Dynamic Range and Extremely High Sensitivity Using Feed-Forward Auto-Bias Adjustment”, IEEE Journal of Solid-State Circuits, Vol. 30, No. 9, pp. 991–997, September 1995).
On the other hand, an amplifier circuit having a signal detection function which notifies about the presence/absence of an optical signal input has been known. A signal detection circuit of this amplifier circuit includes an amplitude detection circuit for detecting an output amplitude of an amplifier and a comparator for comparing an output signal of the amplitude detection circuit and a predetermined reference voltage.
In general, an amplifier in an optical detector has a large gain, and therefore, large noise is sometimes caused in an amplifier output, for example, when no signal is input to the amplifier. In view of such, according to the second conventional technique, a variation that occurs in a comparator output of a signal detection circuit is masked by a monostable multivibrator and an AND gate such that an effect of impulse noise, which occurs in an amplifier output when no optical signal is input or when an optical signal input abruptly changes, is eliminated to prevent a malfunction of the signal detection circuit (Japanese Unexamined Patent Publication No. 5-191354). According to the third conventional technique, a variation that occurs in a comparator output of a signal detection circuit is masked by a delay circuit and an AND gate (Japanese Unexamined Patent Publication No. 10-112689).
Assuming that an amplifier circuit having a signal detection function which is obtained by combining the first conventional technique and the second or third conventional technique is provided, an output signal of an amplifier which receives an output of a reference voltage generation circuit is supplied to a signal detection circuit, and a variation which occurs in a comparator output of the signal detection circuit is masked by a monostable multivibrator or delay circuit and an AND gate.
However, in the case where a signal input to the amplifier circuit abruptly changes, e.g., immediately after the start of inputting of an optical signal or immediately after the stoppage of inputting of the optical signal, a transient response of a low frequency occurs in the output voltage of the reference voltage generation circuit. This transient response has a frequency lower than that of an original signal component, and a long time period is spent until a convergence of the variation. Thus, in order to prevent an adverse effect of the transient response on a signal detection result, the time period for masking a variation that occurs in the comparator output of the signal detection circuit needs to be set to be long beforehand and adjusted at an optimum time. This structure is disadvantageous in integration and size reduction.